Erosion

As we know, the continental ice-sheets consist of large masses of glacial ice covering an extensive area where the surface features are buried under them. Despite their comparatively sluggish movement, the ice-sheets are also capable of eroding the solid bedrock.

The erosion of the bed-rock is accomplished mainly by plucking and rasping (abrasion). Mass-wasting process, which plays a significant role in case of Valley glacier is not at all responsible for supplying rock debris to the ice-sheets.

This is mainly due to the fact that the ice-sheet starts moving only when it becomes sufficiently thick and at this stage, its upper surface is usually above the mountain peaks.

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It is only for this reason that the mass-wasting process cannot operate effectively in the erosion by continental ice-sheets. The plucking and rasping process contribute much to the load carried by the ice-sheets. They are carried frozen into the bottom of the ice-sheet or are pushed ahead by it.

Erosional Features Produced by Continental Ice-Sheets

Continental ice-sheets tend to remove the irregularities of the topog­raphy which are encountered in their path and inhibit their movement.

Rock debris entrapped by the glacier or frozen into the bottom of the glacier enables the glacier to abrade the underlying hard bedrocks and produce features like striations, grooves and polished surfaces on the bed rock over which the ice move and on the rock debris carried at the bottom of the ice.

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Crescentic gouges are formed on the bed rock where rock fragments moving in the ice encounter obstructions in their path and eventually chips off pieces of the bed rock.

The chatter marks which are in the form of a series of curved cracks along the direction of glacier movement, are produced when sharp pointed rock fragments dragged over the surface of the bedrock by the moving ice.

It is to be noted that the curvature of the crescentic gouges are opposite to the curvatures of the chatter marks.

The erosional features like striations, grooves, polished surfaces, crescentic gouges and chatter marks, as described in the proceeding paragraph, constitute the small scale features produced due to erosion by the continental ice-sheets.

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The large-scale erosional features produced by the continental ice-sheets include the followings:

(i) Roches moutonnees (These features have already been des­cribed under valley-glacier)

(ii) Crag and tail When very hard rocks like igneous bodies or volcanic plugs are encountered on the path of the moving ice- sheet, they stand as pillars and offer resistance to the How of the ice and retard its erosive action on the rocks behind the obstruc­tions.

This produces a feature in which the obstructing block facing the moving ice is known as crag followed by a sloping lee side made up of softer rocks called tail.

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Apart from the above features, ice-sheets make enormous excavations on the weak bed rocks. In places, the ice-flow is accentuated by the presence of a valley and behave much as a valley- glacier and produce many of the features of valley glaciation.